LDP (described in “LDP Specification”, RFC 5036, October 2007), which is used to establish LSPs (label switched paths) to destinations, typically relies on IGP (e.g., Open Shortest Path First (OSPF) (defined in “OSPF Version 2”, STD 54, RFC 2328, April 1998), Intermediate system to Intermediate system (IS-IS) (defined in “Intermediate system to intermediate system intra-domain-routing routine information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode Network Service (ISO 8473)” ISO standard 10589, 1992), etc.) to provide the underlying routing information (e.g., the cost metric between the hops in the network). Even though LDP typically relies on the IGP protocol, they are independent from each other. IGP may be operational on a link prior to LDP becoming operational on that link which can lead to packet loss.
The request for comments (RFC) 5443, “LDP IGP Synchronization”, March 2009 (hereinafter “LDP IGP Synchronization”) describes a mechanism to discourage links from being used for IP forwarding if LDP is not fully operational on that link. RFC 5443 describes that when LDP is not fully operational on a given link (e.g., all the label bindings have not yet been exchanged), IGP will advertise the link with maximum cost to discourage traffic from being sent over the link. When LDP on the link becomes operational (e.g., all label bindings have been exchanged), IGP advertises the link with the correct cost.
On broadcast links (more than one LDP/IGP peer on the same link), IGP advertises a common cost to the broadcast link, rather than a separate cost to each peer. A broadcast pseudo-node may be implemented in a network to reduce the number of links in the shortest path first (SPF) (e.g., open shortest path first (OSPF), constrained shortest path first (CSPF)) database of each network element. Each network element forms adjacencies with the broadcast pseudo-node and advertises a link and cost to the broadcast pseudo-node and the broadcast pseudo-node advertises a link and cost to each network element.
For broadcast links, the RFC 5443 “LDP IGP Synchronization” describes that the mechanism can be applied to the link as a whole and not an individual peer. Advertising maximum cost to the link as a whole may cause sub-optimal traffic diversion and/or black-holing of traffic (e.g., VPN traffic).